This invention relates to heat treatment of metal objects and more particularly, the heat treatment of such metal objects as springs, small metal parts, spring components, precision parts, and the like.
The heat treatment of metal objects, such as pressed metal parts and stampings, for the purpose of hardening, tempering, normalizing, annealing, or stress relieving, often results in distortion due to the relief of residual stresses. In order to prevent or minimize, as far as possible, such undesirable distortion, it is known to place such objects in various holding devices such as fine sand or to clamp them into the heat treatment furnace.
One of the earliest known methods of treating steel springs was proposed by Smith in U.S. Pat. No. 5,979, who suggested the use of jigs to hold such springs. Still another approach was suggested by Wilson in U.S. Pat. No. 2,989,428, in which such metal parts are first placed in a metal cannister and plaster of Paris is poured over the objects to hold them in place. The parts are then held by the hardened plaster of Paris and subjected to heat. Still a number of other suggestions have been made to coat metal parts with a glass frit for heat treating purposes.
The use of a fixture to hold individual metal parts is very expensive and a time consuming task, and has proved unacceptable over the years.
The heat necessary to produce molten glass to thereby form a hard and protective coating precludes the use of this method in connection with nonferrous springs such as copper or the like. This is because the temperature of molten glass is often higher than that to which nonferrous material, such as copper, would be subjected for purposes of heat treatment. In addition, and in particular for the treatment of nonferrous springs, it is important that the heat treatment be both rapid and uniform and that the drop in temperature be equally uniform.
In this connection, the method suggested by Wilson is equally inappropriate to nonferrous springs. Plaster is not a refractory material and does not conform to the definition of ceramic, as will be more fully discussed hereinafter. Nonferrous springs are frequently treated at a temperature in excess of 1200.degree.F and plaster cannot sustain such temperatures. Furthermore, Wilson's method does not truly suspend the object to be treated in the ceramic material. Wilson is believed to propose that such objects be embedded at the base of the plaster and be supported during hardening by a metal tray so that the objects are not truly suspended therein. This thereby reduces the ability of the material to equally restrain metal parts. In addition, Plaster of Paris passes slowly through a semi-liquid stage so that it is difficult to set and hold objects in a suspended state within the mixture.